This application relates to the general field of turbine generators and, more particularly, a cooling system therefor.
The generators used in turbogenerator systems usually employ rotors comprising rare earth magnet shafts rotating at very high speed, on the order of 20,000 rpm to 100,000 rpm, within a generator stator. The stator comprises copper windings. Due to the high strength of the magnets and high operating speed, the generators are able to produce very high output power densities (defined as the ratio of power output to generator volume). The inventor recognized that, associated with the power density is a large amount of heating of both the generator stator and the rotor, and that this heat can damage the generator windings and demagnetize the rotor if it is not effectively removed from the generator.
The inventor recognized the disadvantage of fluid cooling generators is that it does not cool the generator rotor shaft, is fairly complex, requiring a circulating pump and a radiator, and it also has the potential to leak fluid and cause damage to the system.
Air-cooled systems have also been used successfully. In one such system the generator stator is pressed into a finned housing. Engine inlet air is then sucked over the fins, thus removing heat from the generator, but directing the heated air into the engine inlet.
In a first aspect, the present invention provides a turbogenerator cooling system, comprising:
(a) a turbogenerator inlet;
(b) a hood section having a front end, a rear end adjacent said turbogenerator inlet, a substantially cylindrical outer surface extending from said front end to said rear end, a substantially cylindrical inner surface extending from said front end to said rear end, and holes adjacent said rear end extending through said hood section;
(b) external fins extending generally axially from front to rear along said outer cylindrical surface of said hood section;
(c) a shroud enclosing said hood section and said external fins;
(d) internal fins extending generally from front to rear along said inner cylindrical surface of said hood section;
(e) a stator press-fit within said hood section in contact with said internal fins;
(f) a permanent magnet rotor shaft rotatably mounted within said stator; and
(g) a fan mounted on said permanent magnet rotor shaft for rotation therewith arranged to draw cooling air (1) rearwardly along said external fins between said hood section and said shroud, (2) inwardly through said holes adjacent the rear end of said hood section, and (3) forwardly away from said turbogenerator inlet along said internal fins between said hood section and said stator.
In another aspect, the invention provides a method of making a turbogenerator cooling system, comprising the steps of:
(a) providing a turbogenerator inlet;
(b) providing a hood section having a front end, a rear end adjacent said turbogenerator inlet, a substantially cylindrical outer surface extending from said front end to said rear end, a substantially cylindrical inner surface extending from said front end to said rear end, and holes adjacent said rear end extending through said hood section;
(b) providing external fins extending generally axially from front to rear along said outer cylindrical surface of said hood section;
(c) providing a shroud enclosing said hood section and said external fins;
(d) providing internal fins extending generally from front to rear along said inner cylindrical surface of said hood section;
(e) press-fitting a stator within said hood section in contact with said internal fins;
(f) mounting a permanent magnet rotor shaft rotatably within said stator; and
(g) mounting a fan on said permanent magnet rotor shaft for rotation therewith arranged for drawing cooling air (1) rearwardly along said external fins between said hood section and said shroud, (2) inwardly through said holes adjacent the rear end of said hood section, and (3) forwardly away from said turbogenerator inlet along said internal fins between said hood section and said stator.
The turbogenerator system cooling system of the present invention comprises a cylindrical heat sink with generally axially extending fins on both the outside and the inside of a hoop section. The hoop section is solid except for holes adjacent the rear end thereof through which cooling air passes to the inside of the hoop section. The generator stator is press-fit into the heat sink with its external surface in contact with the internal fins. The entire assembly is shrouded by a close-fitting cylindrical shroud. The generator rotor is fitted with a small fan for directing the heated air away from the inlet of the engine.
When the turbogenerator rotates, air enters the heat sink at the front of the generator between the hoop section and the shroud and passes along the external fins to the rear of the generator. It then flows through the holes in the hoop section adjacent the rear end thereof, and impinges on the stator winding end-turns. Some of the air then passes forward between the inside of the hoop section and the external surface of the stator along the internal fins. An alternate flow path for some of the air once it has passed through the hoop section is to move forward through the gap between the rotor and the stator to thus cool the rotor hollow sleeve and permanent magnet shafts. The amount of air flowing over the rotor can be adjusted by modifying the size of the space between the internal fins. The air is finally drawn out by the fan and directed away from the engine inlet by suitable ductwork.
The high speed of the fan allows it to create large pressure heads and move large volumes of air. This produces high velocity flow in the heat-sink that in turn results in high heat transfer coefficients. Furthermore, the air, which impinges on the stator end-turns, is very effective at removing heat.
Having fins on both the outside and the inside of the hoop section allows large heat transfer areas to be obtained while using low fin aspect ratios. This low fin aspect ratio allows the heat sink to be extruded which makes its construction very cost effective.
The invention has the following advantages:
The system of the invention is capable of removing a large amount of heat in a small volume, which in turn allows the generator to remain compact thereby reducing the amount of material and the cost of those materials used in the construction of the generator.
The generator rotor is directly cooled thereby improving the generator""s efficiency by increasing the magnetic field strength of the shaft.
The system is very simple requiring no additional moving parts, control systems, or fluids.
Ducting the generator cooling air away from the engine inlet minimizes heating of the air entering the turbogenerator. This improves the net efficiency and increases the power of the turbogenerator system, even when the power consumption of the fan is accounted for.